The subject invention pertains generally to electronic communications and specifically to communications systems entailing the transmission of a digital data modulated carrier signal through a medium polluted with harmonic signals.
The transmission of digital data by carrier signal through some medium may entail discriminating against harmonic signals which pollute the medium in order to properly detect and demodulate the carrier signal to retrieve the data. An example of such a system is the use of electric power systems as a communications channel for data which allows utility customer loads to be monitored for accounting purposes and controlled for load management from a central remote site. Aside from the broadband and impulse noise found on a power system which pose a hostile environment for communication signals, harmonic noise consisting of harmonics of the power system fundamental frequency (60 hertz in the United States and 50 hertz in European countries) must be obviated if a meaningful communication signal is to be transmitted and received. A very effective way for solving this problem is explicated in U.S. Pat. No. 3,368,036 entitled "Demultiplexing and Detecting System For Predicted Wave Phase-Pulsed Data Transmission System" in which is described a commercial system known as Kineplex developed by the assignee of the subject patent application for transmitting a plurality of carrier signals at different equally spaced frequencies in a frequency division multiplexed arrangement between a common transmitter and receiver while preventing any cross-talk among the various channels. This is accomplished by setting the baud period (duration of single period over which the carrier signal characteristic, e.g. phase, remains constant for defining one or more bits) equal to the reciprocal of the frequency separation between adjacent harmonics. The well known resultant (sin x)/x function, where x=.pi.T(F-F.sub.c) radians, representative of the spectral density versus frequency function in the frequency domain for a pulsed carrier signal (of baud period T) at a particular carrier frequency F.sub.c, has nulls at all of the other carrier signal frequencies. Thus a receiver detector which integrates the DC signal developed from the carrier signal indicative of the data for a period equal to that of the baud (and consequently having a matching frequency response) passes only the carrier signal frequency of interest while rejecting all of the others. If all of these undesired carrier signal frequencies vis-a-vis the particular carrier signal frequency of interest are analogized to the power signal harmonic frequencies found on a power system, it becomes readily apparent that the Kineplex technique for discriminating against equally spaced signal frequencies is directly applicable to the problem of transmitting a communication signal over a power system polluted with harmonic noise.
Although the evolving field of power system communications does now employ the aforedescribed Kineplex technique (e.g. see U.S. Pat. Nos. 4,109,204 and 4,012,734), it is to be noted that all of the systems proposed to date center the carrier signal either on a power frequency harmonic or between two adjacent harmonics, thereby failing to take advantage of the disparate magnitudes exhibited by power system harmonics in the discrimination process.
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved means for transmitting communication signals indicative of digital data through a medium polluted with harmonic signals.
It is a further object of the present invention to provide such a new and improved means wherein the adjacent harmonics are of dissimilar magnitude.
It is still a further object of the present invention to provide such a new and improved means wherein the medium is an electric power system and the harmonics are multiples of the power system frequency.
The foregoing objects as well as others and the means by which they are achieved through the present invention may best be appreciated by referring to the Detailed Description of the Invention which follows together with the appended drawings.